1. The Field of the Invention
The present invention generally relates to electrical circuits, and more specifically, relates to electrical circuits for improving bias connections within electrical circuits adapted to accommodate alternating current and direct current signals.
2. The Relevant Technology
Fiber optics are increasingly used for transmitting voice and data signals, such as telecommunication signals, broadcast programming signals, multimedia signals, or the like. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Additionally, light signals are resistant to electromagnetic interference that would otherwise interfere with electrical signals. Light also provides a more secure signal because it does not emanate the type of high frequency components often experienced with conductor-based electrical signals. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper conductors.
The light signals are conducted using the principles of refraction and reflection to effectively trap the light signal within an interior of optical fibers. Because the light signals are trapped within a particular fiber, many fibers can be included in a single cable without concern about interference from the light signals carried by nearby fibers. Optical fibers also have the property of strongly rejecting interference that would otherwise be caused by radio frequencies and electromagnetic radiation. These characteristics make optical fibers ideally suited for many applications.
While optical communications provide a number of advantages, the use of light as a transmission medium presents a number of implementation challenges. In particular, the data carried by a light signal must be converted to an electrical format when received by a device, such as a network switch. Conversely, when data is transmitted to the optical network, it must be converted from an electronic signal to a light signal. A number of protocols define the conversion of electrical signals to optical signals and transmission of those optical signals. For instance, one protocol is implemented using a transceiver module at both ends of a fiber optic cable. Each transceiver module typically contains laser transmitter circuitry capable of converting electrical signals to optical signals, and an optical receiver capable of converting received optical signals back into electrical signals. The laser transmitter circuitry includes a laser driver that causes a laser diode of the transceiver to generate the optical signal representation of the electrical signals. Often, a flexible circuit connects the printed circuit board (PCB) containing the laser driver to the laser diode, or more generally an optical assembly containing the laser diode.
To operate correctly, the laser diode of the transceiver is supplied with both a controlled direct current (DC) bias current and an alternating current (AC) modulation current; the DC bias current allowing the laser diode to properly respond to the AC modulation. Various manners are known to combine AC and DC signals, however, optical networks have specific size, speed, and voltage requirements that limit the applicability of typical techniques. For instance, typical Bias T circuits used to combine AC and DC signals are impractical for use in small form-factor pluggable transceivers, such as those used in optical communication systems. Traditional Bias T circuits use large inductors that are impractical for use in a transceiver because of the size of the components involved. In addition, when a Bias T circuit is used, impedance matching issues associated with signal reflection at the laser diode arise because the inductor is located remotely from the laser diode.
Alternative transceiver circuits utilize resistive dividers to control the AC and DC signals delivered to the laser diode. Unfortunately, resistive dividers lead to a tradeoff between inefficient delivery of AC modulation signals to the laser diode and the use of large resistors in the DC bias chain, which in turn requires large supply voltages and results in a transceiver with high power consumption.
Effects of the above are heightened when the transceiver modulates optical signals at a high rate. In high speed transceiver designs, 10 Gigabits/s, or the like, microwave design considerations often require the placing of matching resistors or impedances very close to the laser diode. In many cases, the flexible circuit connecting the PCB containing the laser driver and the optically assembly includes impedance-controlled lines for the high-speed signals. To perform series matching, the matching resistors or impedances are placed on the flexible circuit at the end near the laser diode and the combined AC modulation signal and DC bias circuit flows through this resistor or impedance and the laser diode, as illustrated in FIG. 1.
As shown, the circuitry 10 includes a laser driver 12 that supplies high-speed electrical modulation needed for a laser diode 16. To allow proper operation of the laser diode, circuitry 10 includes DC bias circuit 14. Impedance matching resistors 18a and 18b are located at the end of lines 20a and 20b near laser diode 16 to prevent loss and signal distortion associated with signal reflection. In this configuration, in the series resistance the bias chain of bias circuit 14 must be made large enough that the portion of the AC modulation signals being lost in DC bias circuit 14 is relatively small. Since the resulting resistors in the bias chain, resistors 22, 18a, 18b, and 24, are relatively large, the voltage drop through this network can be relatively large.
As a consequence of the above, many existing circuits require voltages in excess of 5V typically provided by an external voltage source. Obtaining these increased voltages is difficult for transceivers with 5V external supplies, the typical supply voltage provided to small form-factor pluggable transceivers. Furthermore, the impedance matching to bias circuit 14 is often poor and leads to reflections that degrade the optical output signal generated by laser diode 16.
It would be an advance, therefore, to provide systems and devices that allow for the combining of AC and DC signals, while eliminating the need for internally increasing the supply voltage to accommodate for voltage losses in the transceiver circuitry and maintaining the quality of the AC modulation signals delivered to an optical assembly that generates the light signals propagated along optical fibers.